>>538942052
There's no reason the setting can't have new areas explored, one of the literal only things we know is that the plot begins by Nagito helping you reach a boat. They could be off of Jabberwock Island in under an hour of gameplay for all we know.
The cast isn't locked to the existing 16 used the same ways either. Just by having ones that survived to the end die early and ones that died early survive a long time, they can completely flip the cast on it's head and have entirely new dynamics. We know almost nothing about fat Togami at all. This is before even considering that... they can find new people the same way some routes in The Hundred Line do things like have Eva join the group.